Methods and apparatus for preventing atrial arrhythmias by overdrive pacing multiple heart tissue sites using an implantable cardiac stimulation device

ABSTRACT

Techniques are described for pacing multiple sites in a patient&#39;s heart using overdrive pacing the heart using a pacemaker including techniques where the overdrive pacing rate only increases when at least two intrinsic beats are detected within a determined search period. In one specific technique, an increase in the pacing rate occurs only if two P-waves are detected within X cardiac cycles. In another specific technique, the overdrive pacing rate is increased only if at least two P-waves are detected within a block of N cardiac cycles. In both techniques, the overdrive pacing rate is decreased if no increase has occurred in the last Z cardiac cycles. By increasing the overdrive pacing rate only in response to detection of at least two P-waves within a determined number of cardiac cycles, an excessively high overdrive pacing rate is avoided. Other techniques are described for adaptively adjusting overdrive pacing parameters so as to achieve a determined target degree of pacing of, for example, 95% paced beats. By adaptively adjusting overdrive parameters to maintain a target degree of pacing, the average overdrive pacing rate is minimized while still maintaining a high number of paced beats, thereby reducing the risk of a tachyarrhythmia occurring within the patient.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/548,116, filed Apr. 12, 2000, now U.S. Pat. No. 6,510,342.

FIELD OF THE INVENTION

The invention generally relates to implantable cardiac stimulationdevices such as pacemakers, and in particular, to techniques foroverdrive pacing heart tissue to prevent or terminate dysrhythmia.

BACKGROUND OF THE INVENTION

A dysrhythmia is an abnormal heart beat pattern. One example of adysrhythmia is a bradycardia wherein the heart beats at an abnormallyslow rate or where significant pauses occur between consecutive beats.Other examples of dysrhythmias include tachyarrhythmias where the heartbeats at an abnormally fast rate, e.g., atrial tachycardia where theatria of the heart beat abnormally fast.

One technique for preventing or terminating dysrhythmias is to overdrivepace the heart where an implantable cardiac stimulation device, such asa pacemaker or implantable cardioverter defibrillator (ICD), applieselectrical pacing pulses to the heart at a rate somewhat faster than theintrinsic heart rate of the patient. For bradycardia, the implantablecardiac stimulation device may be programmed to artificially pace theheart at a rate of 60 to 80 pulses per minute (ppm) to thereby preventthe heart from beating too slow and to eliminate any long pauses betweenheart beats. To prevent tachyarrhythmias from occurring, the implantablecardiac stimulation device artificially paces the heart at a rate of atleast five to ten pulses per minute faster than the intrinsictachyarrhythmia heart rate of the patient. In other words, a slightartificial tachycardia is induced and maintained in an effort to preventan actual tachycardia from arising.

It is believed that overdrive pacing is effective in at least somepatients for preventing or terminating the onset of an actualtachycardia for the following reasons. A normal, healthy heart beatsonly in response to electrical pulses generated from a portion of theheart referred to as the sinus node. The sinus node pulses are conductedto the various atria and ventricles of the heart via certain, normalconduction pathways. In some patients, however, additional portions ofthe heart also generate electrical pulses referred to as “ectopic”pulses. Each pulse, whether a sinus node pulse or an ectopic pulse, hasa refractory period subsequent thereto during which time the hearttissue is not responsive to any electrical pulses. A combination ofsinus pulses and ectopic pulses can result in a dispersion of therefractory periods which, in turn, can trigger a tachycardia. Byoverdrive pacing the heart at a uniform rate, the likelihood of theoccurrence of ectopic pulses is reduced and the refractory periodswithin the heart tissue are rendered uniform and periodic. Thus, thedispersion of refractory periods is reduced and tachycardias triggeredthereby are substantially avoided.

Thus, it is desirable with patients prone to tachyarrhythmias to ensurethat most beats of the heart are paced beats, as any unpaced beats maybe ectopic beats. A high percentage of paced beats can be achievedsimply by establishing a high overdrive pacing rate. However, a highoverdrive pacing rate has disadvantages as well. For example, a highoverdrive pacing rate may be unpleasant to the patient, particularly ifthe artificially-induced heart rate is relatively high in comparisonwith the heart rate that would otherwise normally occur. A high heartrate may also cause possible damage to the heart or may trigger moreserious dysrhythmias, such as a ventricular fibrillation.

A high overdrive pacing rate may be especially problematic in patientssuffering from heart failure, particularly if the heart failure is dueto an impaired diastolic function. A high overdrive pacing rate mayactually exacerbate heart failure in these patients. Also, a highoverdrive pacing rate may be a problem in patients with coronary arterydisease because increasing the heart rate decreases diastolic time anddecreases perfusion, thus intensifying ischemia. Also, the need to applyoverdrive pacing pulses operates to deplete the implantable cardiacstimulation device's power supply, perhaps requiring frequent surgicalreplacement of the power supply. Typically, the power supply is locatedwithin the implantable cardiac stimulation device and thus this requiressurgical replacement of the implantable cardiac stimulation device.

Problems associated with overdrive pacing are particularly severe forcertain aggressive overdrive techniques which trigger an increase in thepacing rate based upon detection of a single intrinsic heart beat. Withsuch techniques, a significant increase in the pacing rate is triggeredby detection of a single intrinsic heart beat so as to promptly respondto the occurrence of a high rate tachycardia, such as an SVT. As aresult, even in circumstances where a high rate tachycardia has notoccurred, the detection of a single intrinsic heart beat can cause asignificant increase in the overdrive pacing rate, which may be reducedonly gradually. If a second intrinsic heart beat is detected before theoverdrive pacing rate has been gradually lowered to a standard overdrivepacing rate, a still further increase in the pacing rate occurs. As canbe appreciated, the foregoing can cause the overdrive pacing rate toincrease significantly, perhaps to 150 ppm or more, even though a highrate tachycardia has not occurred. The aforedescribed problems areaddressed by a copending, commonly-assigned patent application to Florioet al., entitled “Methods and Apparatus for Overdrive Pacing HeartTissue using an Implantable Cardiac Stimulation Device,” the contents ofwhich are incorporated herein by reference in their entirety.

An alternative approach to reduce atrial arrhythmias is described inU.S. Pat. No. 5,403,356 to Hill et al. (the Hill patent). The Hillpatent describes placing at least two electrodes in the atrium,preferably in the triangle of Koch and/or an area of prolonged effectiverefractory period elsewhere in the atrium. Pacing pulses are thenapplied to the multiple electrodes either simultaneously or separated bya short delay.

While each approach offers benefits in avoiding incidences ofarrhythmia, e.g., atrial arrhythmia, it is believed that treatment canbe further improved by combining selected aspects of both approaches toachieve an improved result. Accordingly, the present invention isdirected to that end.

SUMMARY

The present invention is directed toward a method and apparatus forreducing the incidence of atrial arrhythmias by using an overdrivealgorithm to determine the application of stimulation pulses to two ormore electrodes distributed among multiple sites in a patient's heart,e.g., in the atria. In accordance with a first preferred embodiment, theelectrodes may be distributed within a single atrium, e.g., the rightatrium, of the patient's heart. Alternatively, a first electrode may beplaced in the right atrium and a second electrode may be placed in thecoronary sinus or the left atrium or multiple electrodes may be placedproximate to the left atrium.

In accordance with a first aspect of invention, a method is provided foroverdrive pacing multiple sites in a patient's heart using animplantable cardiac stimulation device wherein an increase in anoverdrive pacing rate is performed only in response to detection of atleast two intrinsic beats within a predetermined search period.Initially, an overdrive pacing rate is determined and the heart is pacedat the overdrive pacing rate. Intrinsic heart beats arising duringoverdrive pacing are detected. If at least two intrinsic heart beats aredetected within a first predetermined search period, then the overdrivepacing rate is increased by a predetermined rate increment. If at leasttwo intrinsic heart beats are not detected within a second predeterminedsearch period, the overdrive pacing rate is decreased by a predeterminedrate of decrements. By increasing the overdrive pacing rate only inresponse to the detection of at least two intrinsic heart beats withinthe first predetermined search period, an excessively high heart rate isavoided as might otherwise occur if an overdrive pacing rate increasewere based upon detection of only a single intrinsic heart beat.

In an exemplary embodiment, the first predetermined search periodextends for X number of cardiac cycles following detection of a firstintrinsic beat wherein X is in the range of eight to forty cycles. Thesecond predetermined search period Z is also within the range of eightto forty cardiac cycles. If, after detection of a first intrinsic heartbeat, a second heart beat is detected within X cardiac cycles, then theoverdrive pacing rate is increased by Y ppm wherein Y is five, ten,fifteen, twenty or twenty-five. If Z cardiac cycles occur without a rateincrease, then the rate is decreased by an amount W ppm per cardiaccycle wherein W is one, two, three, four or five.

In a second exemplary embodiment, the first predetermined search periodis N consecutive cardiac cycles, wherein N is, for example, ten. Thus,if there are at least two intrinsic heart beats within a set of Nconsecutive cardiac cycles, the overdrive pacing rate is increased.Otherwise, the overdrive pacing rate is decreased. With the secondembodiment, it is easy to program a minimum percentage of paced beats.For example, to attain at least a minimum of 90% paced beats, N is setat ten. If fewer than 90% of the beats are paced beats (i.e., at leasttwo beats out of every ten beats are intrinsic beats), the overdrivepacing rate is increased; otherwise it is decreased. This provides afeedback loop which maintains the pacing rate at a rate sufficient toprovide about 90% paced beats on the average.

In still other embodiments, the implantable cardiac stimulation devicemay periodically suspend overdrive pacing to permit detection of threeconsecutive intrinsic heart beats. The intrinsic heart rate iscalculated based upon those three heart beats and overdrive pacingresumes at a rate corresponding to the intrinsic heart rate. In anotherembodiment, the implantable cardiac stimulation device periodicallydetermines the intrinsic atrial rate and compares the atrial rate withthe current overdrive pacing rate. If the difference between the atrialrate and the overdrive pacing rate exceeds a predetermined thresholdamount, the implantable cardiac stimulation device adjusts the overdrivepacing rate to equal the atrial rate. Otherwise, the implantable cardiacstimulation device continues to pace at the current overdrive pacingrate.

In accordance with a second aspect of the invention, a method isprovided for adaptively varying overdrive pacing characteristics so asto achieve a predetermined degree of pacing. Overdrive pacing pulses areapplied to the heart in accordance with programmed values specifyingoverdrive pacing characteristics. An actual degree of pacing resultingfrom the overdrive pacing pulses is determined. The programmed valuesare then varied based upon the degree of pacing resulting from theoverdrive pacing pulses.

In an exemplary embodiment of the second aspect of the invention,overdrive pacing is performed by periodically determining an intrinsicatrial rate, then pacing the heart at a rate equal to the intrinsic rateplus an additional overdrive pacing margin. The overdrive pacing marginis thereafter selectively increased or decreased so as to maintain theactual degree of pacing at about 95% paced beats. To this end, theoverdrive pacing margin, which may initially be set to five beats perminute above the intrinsic heart rate, may be incrementally increased ordecreased to maintain the percentage of paced beats at the target rateof about 95%.

In a second exemplary embodiment, overdrive pacing is performed inaccordance with a dynamic atrial overdrive technique which operates toperiodically increase a pacing cycle length (i.e., to decrease thepacing rate) to permit detection of intrinsic paced beats. The pacingcycle length is automatically extended every N_(MAX) cardiac cycles by apredetermined amount. Initially, N_(MAX) may be, for example, tencycles. In accordance with the invention, the value for N_(MAX) isperiodically increased or decreased in accordance with the actual degreeof pacing so as to maintain the actual degree of pacing at about 95%paced beats. Hence, possible disadvantages associated with increasingthe overdrive pacing rate in response to detection of only a singleintrinsic beat are substantially avoided and the average overdrivepacing rate is kept reasonably low. Other programmable values definingthe dynamic atrial overdrive algorithm may also be adaptively varied inaccordance with the actual degree of pacing.

Hence, with the second aspect of the invention, overdrive pacingcharacteristics are adaptively varied so as to reduce the averageoverdrive pacing rate while still maintaining, on the average, thetarget percentage of paced beats. The degree of risk or discomfort tothe patient resulting from overdrive pacing is thereby minimized and thelongevity of the power supply of the implantable cardiac stimulationdevice is increased while still achieving a sufficiently high percentageof paced beats to reduce the risk that a naturally occurringtachyarrhythmia will occur within the patient.

In a significant aspect of the present invention, the aforedescribedoverdrive algorithm may be performed via multiple sites in a patient'sheart. In a first embodiment, two or more electrodes are distributed inthe atria, e.g., the right atrium of the patient's heart, and a combinedsignal received from the electrodes is used as input to theaforedescribed overdrive algorithm. The algorithm then triggers a singlepulse generator that is used to simultaneously drive multiple electrodesto simultaneously stimulate multiple sites in the patient's heart.Alternatively, multiple pulse generators can be used to individuallydrive the multiple electrodes, thus enabling a programmed time delaybetween stimulation of each of the sites in the patient's heart.Additionally, cardiac tissue depolarizations can be individually sensedfrom each of the electrodes and the sensed signal indicating the highestheart rate can be used as input to the overdrive algorithm. Again,pacing pulses can be applied either simultaneously or staggered with aprogrammed time delay to multiple sites in the patient's heart.

Apparatus embodiments of the invention are also provided. Other aspects,features, and advantages of the invention will be apparent from thedetailed description which follows in the combination with the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram illustrating an implantable cardiacstimulation device, a pacemaker/defibrillator, internally configured inaccordance with the invention, and connected to a patient's heart.

FIG. 1B is a schematic diagram of an alternative embodiment of a portionof the implantable cardiac stimulation device of FIG. 1A showing the useof multiple pulse generators for individually driving each of theimplanted electrodes.

FIG. 1C is a schematic diagram of an alternative embodiment of a portionof the implantable cardiac stimulation device of FIG. 1A showing the useof a pulse delay circuit in conjunction with a pulse generator forindividually driving each of the implanted electrodes.

FIG. 1D is a schematic diagram of an alternative embodiment of a portionof the implantable cardiac stimulation device of FIG. 1A showing the useof multiple sense amplifiers for individually sensing depolarizationsfrom the multiple electrodes.

FIG. 2 is a timing diagram illustrating paced beats and unpaced beatswithin the heart of FIG. 1A.

FIG. 3 is a flow chart illustrating an overdrive pacing method whereinan overdrive pacing rate is increased only if at least two intrinsicevents are detected within X cardiac cycles of one another.

FIG. 4 is a flow chart illustrating an overdrive pacing method whereinan overdrive pacing rate is increased only if at least two intrinsicheart beats are detected within a block of N consecutive cardiac cycles.

FIG. 5 is a flow chart illustrating a method for adaptively varyingprogrammable values defining overdrive pacing characteristics so as tomaintain a target degree of pacing.

FIG. 6 is a flow chart of the method of FIG. 5 configured for adaptivelyvarying an overdrive pacing rate.

FIG. 7 is a flow chart illustrating a method for adaptively modifyingthe automatic pacing cycle length adjustment to maintain a target degreeof pacing.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed toward a method and apparatus forreducing the incidence of atrial arrhythmias by using an overdrivealgorithm to determine the application of stimulation pulses to two ormore electrodes distributed among multiple sites in a patient's heart,e.g., in the atria. In accordance with a first preferred embodiment, theelectrodes may be distributed within a single atrium, e.g., the rightatrium, of the patient's heart. Alternatively, a first electrode may beplaced in the right atrium and a second electrode may be placed in thecoronary sinus or the left atrium. Furthermore, the invention relates totechniques for controlling overdrive pacing to the multiple sites so asto achieve and maintain a target degree of pacing and thus suppresstachycardias. The techniques will first be described with reference toFIGS. 1–4 wherein an overdrive pacing rate is increased only in responseto detection of at least two intrinsic heart beats within somepredetermined number of cardiac cycles. Then, techniques of theinvention will be described with reference to FIGS. 5–7 whereinprogrammable values specifying overdrive pacing characteristics areadaptively varied so as to maintain a target percentage of paced beats.

In FIG. 1A, a simplified block diagram is shown of a dual-chamberimplantable cardiac stimulation device 10 which is capable of treatingboth fast and slow arrhythmias with stimulation therapy, includingcardioversion, defibrillation, and pacing stimulation. While adual-chamber device is shown, this is for illustration purposes only,one of skill in the art could readily eliminate or disable theappropriate circuitry to provide a single-chamber stimulation devicecapable of treating one chamber with cardioversion, defibrillation andpacing stimulation or appropriate circuitry may be added to provide athree or four chamber stimulation device.

To provide atrial chamber pacing stimulation and sensing, theimplantable cardiac stimulation device 10 is shown in electricalcommunication with a patient's heart 12 by way of implantable atrialleads 20 a and 20 b, respectively having atrial tip electrodes 22 a and22 b and atrial ring electrodes 24 a and 24 b. The electrode pairs 22 a,24 a and 22 b, 24 b are preferably distributed among two sites in theright atrium for dual site pacing or one pair is positioned in the rightatrium, e.g., in the patient's atrial appendage, and the second pair ispositioned in the coronary sinus or the left atrium for bi-atrialpacing. For example, a lead designed for placement in the coronary sinusregion could be implanted to deliver left atrial pacing. For a completedescription of a coronary sinus lead, see U.S. patent application Ser.No. 09/196,898, “A Self-Anchoring Coronary Sinus Lead” (Pianca et al.),and U.S. Pat. No. 5,466,254, “Coronary Sinus Lead with Atrial SensingCapability” (Helland), which patents are hereby incorporated herein byreference. Alternatively, both electrode pairs may be located proximateto the left atrium, i.e., both electrode pairs may located in the leftatrium, one electrode pair may be located in the left atrium with thesecond electrode pair located in the coronary sinus, etc. It is believedthat by pacing, either simultaneously or staggered, to multiple (two ormore), e.g., atrial, sites in the patient's heart, spontaneousdepolarizations of ectopic foci can be suppressed and thus the potentialfor tachycardia, e.g., atrial tachycardia, can be minimized.

The implantable cardiac stimulation device 10 is also shown inelectrical communication with the patient's heart 12 by way of animplantable ventricular lead 30 having, in this embodiment, aventricular tip electrode 32, a ventricular ring electrode 34, a rightventricular (RV) coil electrode 36, and an SVC coil electrode 38.Typically, the ventricular lead 30 is transvenously inserted into theheart 12 so as to place the RV coil electrode 36 in the rightventricular apex, and the SVC coil electrode 38 in the superior venacava. Accordingly, the ventricular lead 30 is capable of receivingcardiac signals and delivering stimulation in the form of pacing andshock therapy to the right ventricle.

The housing 40 (shown schematically) for the implantable cardiacstimulation device 10 includes a connector (not shown) having atrial tipterminals 42 a and 42 b and atrial ring terminals 44 a and 44 b, whichare adapted for connection to the atrial tip electrodes 22 a and 22 band the atrial ring electrodes 24 a and 24 b, respectively. The housing40 further includes a ventricular tip terminal 52, a ventricular ringterminal 54, a right ventricular (RV) shocking terminal 56, and an SVCshocking terminal 58, which are adapted for connection to theventricular tip electrode 32, the ventricular ring electrode 34, the RVcoil electrode 36, and the SVC coil electrode 38, respectively. Thehousing 40 (often referred to as the “can”, “case” or “case electrode”)may be programmably selected to act as the return electrode, or anodealone or in combination with one of the coil electrodes, 36 and 38. Forconvenience, the names of the electrodes are shown next to theterminals.

At the core of the implantable cardiac stimulation device 10 is aprogrammable microcontroller 60 or other processor which controls thevarious modes of stimulation therapy. As is well known in the art, themicrocontroller 60 includes a microprocessor, or equivalent controlcircuitry, designed specifically for controlling the delivery ofstimulation therapy and may further include RAM or ROM memory, logic andtiming circuitry, state machine circuitry, and I/O circuitry. Typically,the microcontroller 60 includes the ability to process or monitor inputsignals (data) as controlled by program code stored in a designatedblock of memory. The details of the design and operation of themicrocontroller 60 are not critical to the present invention. Rather,any suitable microcontroller 60 may be used that carries out thefunctions described herein. The use of microprocessor-based controlcircuits for performing timing and data analysis functions is well knownin the art.

As shown in FIG. 1A, an atrial pulse generator 70 and a ventricularpulse generator 72 generate pacing stimulation pulses for delivery tothe patient's heart by the atrial leads 20 a and 20 b and theventricular lead 30, respectively, via a switch bank 74. The pulsegenerators, 70 and 72, are controlled by the microcontroller 60 viaappropriate control signals, 76 and 78, respectively, to trigger orinhibit the stimulation pulses. The microcontroller 60 further includestiming circuitry that controls the implantable cardiac stimulationdevice's timing of such stimulation pulses.

The switch bank 74 includes a plurality of switches for switchablyconnecting the desired electrodes to the appropriate I/O circuits,thereby providing complete electrode programmability. Accordingly, theswitch bank 74, in response to a control signal 80 from themicrocontroller 60, determines the polarity of the stimulation pulses(e.g., unipolar or bipolar) by selectively closing the appropriatecombination of switches (not shown) as is known in the art.

An atrial (ATR) sense amplifier 82 and a ventricular (VTR) senseamplifier 84 are also coupled to the atrial and ventricular leads 20 and30, respectively, through the switch bank 74 for detecting the presenceof cardiac activity. The switch bank 74 determines the “sensingpolarity” of the cardiac signal by selectively closing the appropriateswitches, as is also known in the art. In this way, the clinician mayprogram the sensing polarity independent of the stimulation polarity.

Each sense amplifier, 82 and 84, preferably employs a low power,precision amplifier with programmable gain and/or automatic gaincontrol, bandpass filtering, and a threshold detection circuit, known inthe art, to selectively sense the cardiac signal of interest. Theautomatic gain control enables the implantable cardiac stimulationdevice 10 to deal effectively with the problem of sensing the lowfrequency, low amplitude signal characteristics of ventricularfibrillation.

The outputs of the atrial and ventricular sense amplifiers, 82 and 84,are connected to the microcontroller 60 which, in turn, inhibit theatrial and ventricular pulse generators, 70 and 72, respectively, in ademand fashion whenever cardiac activity is sensed in the respectivechambers. The sense amplifiers, 82 and 84, in turn, receive controlsignals over signal lines, 86 and 88, from the microcontroller 60 forpurposes of controlling the gain, threshold, polarization charge removalcircuitry (not shown), and the timing of any blocking circuitry (notshown) coupled to the inputs of the sense amplifiers, 82 and 84, as isknown in the art.

For arrhythmia detection, the present invention utilizes the atrial andventricular sense amplifiers, 82 and 84, to sense cardiac signals todetermine whether a rhythm is physiologic or pathologic. As used herein“sensing” is reserved for the noting of an electrical depolarization,and “detection” is the processing of these sensed depolarization signalsand noting the presence of an arrhythmia. The timing intervals betweensensed events (e.g., the P—P and R—R intervals) are then classified bythe microcontroller 60 by comparing them to a predefined rate zone limit(i.e., bradycardia, normal, low rate VT, high rate VT, and fibrillationrate zones) and various other characteristics (e.g., sudden onset,stability, physiologic sensors, and morphology, etc.) in order todetermine the type of remedial therapy that is needed (e.g., bradycardiapacing, anti-tachycardia pacing, cardioversion shocks or defibrillationshocks, also known as “tiered therapy”).

Cardiac signals are also applied to the inputs of an analog to digital(A/D) data acquisition system 90. The data acquisition system 90 isconfigured to acquire intracardiac electrogram signals, convert the rawanalog data into a digital signal, and store the digital signals forlater processing and/or telemetric transmission to an external device102. The data acquisition system 90 is coupled to the atrial andventricular leads, 20 a, 20 b, and 30, through the switch bank 74 tosample cardiac signals across any pair of desired electrodes.

The microcontroller 60 is further coupled to a memory 94 by a suitabledata/address bus 96, wherein the programmable operating parameters usedby the microcontroller 60 are stored and modified, as required, in orderto customize the operation of the implantable cardiac stimulation device10 to suit the needs of a particular patient. Such operating parametersdefine, for example, pacing pulse amplitude, pulse duration, electrodepolarity, rate, sensitivity, automatic features, arrhythmia detectioncriteria, and the amplitude, waveshape and vector of each shocking pulseto be delivered to the patient's heart 12 within each respective tier oftherapy.

Advantageously, the operating parameters of the implantable cardiacstimulation device 10 may be non-invasively programmed into the memory94 through a telemetry circuit 100 in telemetric communication with anexternal device 102, such as a programmer. The telemetry circuit 100 isactivated by the microcontroller via control signal 106. The telemetrycircuit 100 advantageously allows intracardiac electrograms and statusinformation relating to the operation of the implantable cardiacstimulation device 10 in addition to the data contained in the memory 94relating to the interaction of the device with the patient's heart to besent to the external device 102 through an established communicationlink 104. For examples of such devices, see U.S. Pat. No. 4,809,697,entitled “Interactive Programming and Diagnostic System for use withImplantable Pacemaker” (Causey, III et al.) and U.S. Pat. No. 4,944,299,entitled “High Speed Digital Telemetry System for Implantable Device”(Silvian).

In the preferred embodiment, the implantable cardiac stimulation device10 further includes a physiologic sensor 110. Such sensors are commonlycalled “rate-responsive” sensors. The physiological sensor 110 is usedto detect the exercise state of the patient, to which themicrocontroller 60 responds by adjusting the rate and AV delay at whichthe atrial and ventricular pulse generators, 70 and 72, generatestimulation pulses. A common type of rate responsive sensor is anactivity sensor, such as an accelerometer or a piezoelectric crystal,which is mounted within the housing 40 of the implantable cardiacstimulation device 10. Other types of physiologic sensors are alsoknown, for example, sensors which sense the oxygen content of blood,respiration rate and/or minute ventilation, pH of blood, ventriculargradient, etc. However, any sensor may be used which is capable ofsensing a physiological parameter which corresponds to the exercisestate of the patient. The type of sensor used is not critical to thepresent invention and is shown only for completeness.

The implantable cardiac stimulation device 10 additionally includes abattery 114 which provides operating power to all of the circuits shownin FIG. 1A. For the implantable cardiac stimulation device 10, whichemploys shocking therapy, the battery 114 must be capable of operatingat low current drains for long periods of time, and, in the case wherethe pacemaker also performs as a cardioverter/defibrillator, the batterymust also be capable of providing high-current pulses (for capacitorcharging) when the patient requires a shock pulse. The battery 114 mustalso have a predictable discharge characteristic so that electivereplacement time can be detected. Accordingly, the present inventionpreferably employs lithium/silver vanadium oxide batteries, as ispresently true for many such devices.

The implantable cardiac stimulation device 10 further includes a magnetdetection circuitry (not shown) coupled to the microcontroller 60. It isthe purpose of the magnet detection circuitry to detect when a magnet isplaced over the implantable cardiac stimulation device 10, which magnetmay be used by a clinician or patient to perform various functionscontrolling the implantable cardiac stimulation device 10.

As further shown in FIG. 1A, the present invention preferably includesan impedance measuring circuit 120 which is enabled by themicrocontroller 60 by a control signal 122. The known uses for animpedance measuring circuit 120 include, but are not limited to, leadimpedance surveillance during the acute and chronic phases for properlead positioning or dislodgment; detecting operable electrodes andautomatically switching to an operable pair if dislodgment occurs;measuring respiration or minute ventilation; measuring thoracicimpedance for determining shock thresholds; detecting when the devicehas been implanted; measuring stroke volume; and detecting the openingof the valves, etc. The impedance measuring circuit 120 isadvantageously coupled to the switch bank 74 so that any desiredelectrode (including the RV and SVC coil electrodes, 36 and 38) may beused. The impedance measuring circuit 120 is not critical to the presentinvention and is shown for only completeness.

FIG. 1B shows a schematic diagram of an alternative embodiment of aportion of the implantable cardiac stimulation device 10 of FIG. 1Ashowing the use of multiple pulse generators 70 a and 70 b forindividually driving each of the implanted atrial electrode pairs. Eachof the pulse generators 70 a and 70 b are individually controlled by themicrocontroller 60 via control signals 76 a, 76 b. Accordingly, in thisembodiment, the pacing of each site can be individually controlled. Inoperation, this embodiment allows either simultaneous pacing or pacingstaggered between the two sites by a determined time delay, e.g., pacingthe right atrium in advance of the left atrium.

FIG. 1C is a schematic diagram of an alternative embodiment of a portionof the implantable cardiac stimulation device 10 of FIG. 1A showing theuse of a pulse delay circuit in conjunction with a pulse generator forindividually driving each of the implanted atrial electrodes. Thisembodiment performs similarly to that described in reference to FIG. 1Bexcept that a single pulse generator 70 is used in conjunction with apulse delay circuit 106 which is used to stagger the pacing between themultiple sites. Preferably the pulse delay circuit 106 is programmablevia control line 108 to enable the microcontroller 60 to alter thestagger amount.

FIG. 1D is a schematic diagram of an alternative embodiment of a portionof the implantable cardiac stimulation device 10 of FIG. 1A showing theuse of multiple sense amplifiers 82 a and 82 b for individually sensingdepolarizations from the multiple atrial electrode pairs. Accordingly,in this embodiment, the depolarization rate can be individually sensedat each of the sites in the patient's heart 12. In a first case, thedepolarization rate may be the same at each site with a time delayreflecting the conduction speed from the depolarized tissue to eachelectrode. In a second case, the distribution of ectopic foci may resultin a different depolarization rate at each site. In a preferredembodiment, the present invention senses the highest depolarization rateand uses that rate as input to the overdrive algorithm described below.Furthermore, even if the sensed depolarization rates are essentiallyequivalent, it is preferable that the site at which depolarization issensed earliest be the site that is stimulated first. For example, if anatrial tachycardia is caused by ectopic foci in the right atrium, anelectrode in the right atrium will sense the depolarization before anelectrode in the left atrium or coronary sinus. Accordingly, to suppressdepolarizations of the ectopic foci, it may be preferable to stimulateright atrial electrode before stimulating the left atrial electrode.Accordingly, the sensing embodiment of FIG. 1D is preferably used inconjunction with the pulse generator embodiment of either FIG. 1B orFIG. 1C to enable this function.

FIG. 2 illustrates a sequence of pacing pulses 152 administered by theimplantable cardiac stimulation device 10 of FIG. 1A. Each electricalpacing pulse triggers an evoked response 154 representative of anartificially induced heart beat. FIG. 2 also illustrates a singleunpaced beat 156 not preceded by a pacing pulse 152. Unpaced beat 156may be, for example, an ectopic beat caused by a naturally occurringelectrical signal generated within the heart from a location other thanthe sinus node from which normal sinus rhythm heart beats are naturallygenerated. As discussed above, ectopic beats have been found tosometimes trigger tachyarrhythmias and hence it is desirable to minimizethe number of ectopic beats. Accordingly, the implantable cardiacstimulation device 10 of FIG. 1A performs an overdrive pacing algorithmintended to generate overdrive pacing pulses 152 at a sufficiently highrate to minimize the number of unpaced beats 156 without causing anunnecessarily high heart rate. To this end, the implantable cardiacstimulation device 10 determines the actual degree of pacing resultingfrom the overdrive pacing pulses and adaptively modifies the overdrivepacing rate to maintain the actual degree of pacing at about a targetdegree of pacing wherein about 95% of the total beats are paced beats.

FIG. 3 illustrates a technique for overdrive pacing a heart wherein anoverdrive pacing rate is increased only in response to the detection ofat least two intrinsic P-waves occurring within X cardiac cycles of oneanother wherein X is, typically, between eight and forty cardiac cycles.Briefly, the technique is summarized as follows:

-   -   1. Identify a P-wave.    -   2. If another P-wave occurs within X cardiac cycles, increase        the pacing rate by Y bpm.        -   a) X is preferably programmable from about 8 to 40 cardiac            cycles.        -   b) Y is the programmable rate increase and is preferably            programmable to 5, 10, 15, 20 or 25 ppm.    -   3. If Z cardiac cycles occur without a pacing rate increase,        then decrease the pacing rate by W ppm.        -   a) Z is the dwell time before the pacing rate is decreased            and is preferably programmable from 8 to 40 cardiac cycles.        -   b) W is preferably programmable at 1, 2, 3, 4, or 5 ppm.

Additionally, the implantable cardiac stimulation device 10 periodicallysuspends pacing to permit detection of three consecutive P-waves. Atthat time, the sinus rate is computed based upon the detected P-wavesand the overdrive pacing rate is reset to be equal to the sinus rate.

This technique will now be explained more fully with reference to FIG.3. Initially, at step 200, the intrinsic sinus rate is detected bydetecting three consecutive P-waves. At step 204, a current overdrivepacing rate is set to be equal to the detected sinus rate. At step 206,the implantable cardiac stimulation device 10 begins to count the numberof paced cycles (I_(RESET)) since the overdrive pacing rate was setbased upon the intrinsic sinus rate. This count of paced cycles iseventually compared with a rate recalibration value in step 218(described below) and if it exceeds the recalibration value, step 200 isrepeated to detect a new intrinsic sinus rate for resetting of theoverdrive pacing rate.

At step 208, the atria of the heart is paced at the current overdrivepacing rate. At step 210, the number of cycles (I_(CYCLES)) since pacingbegan at the current rate is counted. Note that, I_(RESET) andI_(CYCLES) are initially equal to one another. However, as will bedescribed, the values typically diverge from one another with furtherexecution of the method steps. I_(CYCLES) is eventually compared againsta rate recovery value Z at step 214 (described below) and if I_(CYCLES)exceeds Z, the overdrive pacing rate is decreased.

At step 212, the implantable cardiac stimulation device 10 detects anyintrinsic P-waves. If a P-wave is not detected, processing proceeds tostep 214 wherein the count of paced cycles since pacing began at thecurrent rate (I_(CYCLES)) is compared with the rate recovery value (Z).If I_(CYCLES) exceeds Z, then step 216 is performed wherein the currentoverdrive pacing rate is decreased by a pacing decrement amount W,preferably preset to 1, 2, 3, 4, or 5 ppm. Processing then returns tostep 206 for continued pacing at the newly reduced overdrive pacingrate. Thus, if at least I_(CYCLES) of pacing occurs before detection ofa single intrinsic P-wave, the current overdrive pacing rate is reducedto provide for a rate of recovery. If, at step 214, I_(CYCLES) does notexceed Z, then step 218 is performed wherein the implantable cardiacstimulation device 10 determines whether the count of paced cycles sincethe current rate was originally set (I_(RESET)) exceeds a raterecalibration value (N_(RECALIBRATION)). If I_(RESET) exceedsN_(RECALIBRATION), then step 200 is again executed wherein a new sinusrate is detected and the overdrive pacing rate is reset to the new sinusrate. This ensures that the overdrive pacing rate does not remainsignificantly different from the sinus rate for any extended period oftime. If, at step 218, I_(RESET) does not exceed N_(RECALIBRATION), thenprocessing returns to step 206 for additional pacing at the currentoverdrive pacing rate.

What has been described thus far with respect to FIG. 3 arecircumstances wherein no intrinsic P-waves are detected. Steps performedin response to the detection of P-waves will now be described. Morespecifically, if at step 212 an intrinsic P-wave is detected, then step220 is performed wherein a determination is made as to whether a counthas already begun of the number of overdrive pacing cycles sincedetection of the last detected P-wave. During the first execution ofstep 220 following detection of the first P-wave, the count has not yetbegun and hence processing continues to step 222 wherein the implantablecardiac stimulation device 10 begins to count the number of overdrivepacing cycles since the last detected P-wave (I_(P-WAVE)). Processingthen returns to step 206 for additional pacing at the current overdrivepacing rate while incrementing I_(P-WAVE) (along with I_(RESET) andI_(CYCLES)) with each additional pacing cycle.

If another P-wave is detected at step 212, then execution proceedsthrough step 220 to step 224 wherein I_(P-WAVE) is compared with apacing cycle increment value (X). If I_(P-WAVE) is less than X,indicating that the last two detected intrinsic P-waves are within Xcardiac cycles of one another, then step 226 is performed wherein thecurrent overdrive pacing rate is increased by a predetermined pacingincrement amount (Y) set to, for example, 5, 10, 15, 20, or 25 ppm.Thereafter, pacing continues from step 206 at the new higher overdrivepacing rate. If, however, at step 224, I_(P-WAVE) was found to begreater than X, meaning that the last two detected intrinsic P-waveswere more than X cycles apart, then the overdrive pacing rate is notimmediately increased. Instead, processing proceeds to step 222 whereinI_(P-WAVE) is reset to begin a new count of the number of overdrivepacing cycles since the most recently detected P-wave.

Thus, FIG. 3 is a flow chart illustrating one technique for implementingan overdrive pacing algorithm which, among other features, (1) increasesan overdrive pacing rate if two P-waves are detected within X cardiaccycles of one another, (2) decreases the overdrive pacing rate if a rateincrease does not occur within at least Z cardiac cycles, and (3) resetsthe overdrive pacing rate to be equal to a detected sinus rate everyN_(RECALIBRATION) number of cardiac cycles regardless of the extent towhich the overdrive pacing rate is modified during the interim.

FIG. 4 illustrates a method for controlling overdrive pacing wherein anoverdrive pacing rate is increased only if at least two intrinsic atrialbeats are detected within a block of N consecutive cardiac cycles. Thetechnique of FIG. 4 is summarized as follows:

-   -   1. At the conclusion of a block of N cardiac cycles, the        implantable cardiac stimulation device determines if there is        more than one P-wave in the block of cardiac cycles. If there is        more than one P-wave, the pacing rate is increased by Y ppm.        -   a) Y is a programmable rate increment and is preferably            programmable to 5, 10, 15, 20 or 25 ppm.    -   2. If Z cardiac cycles occur without increasing the pacing rate,        then the pacing rate is decreased by W ppm.        -   a) Z is the dwell time before the pacing rate is decreased            and is preferably programmable from 8 to 40 cardiac cycles.        -   b) W is preferably programmable at 1, 2, 3, 4, or 5 ppm.

Additionally, the implantable cardiac stimulation device 10 periodicallysuspends pacing to detect the intrinsic atrial rate and compare theintrinsic atrial rate with a current overdrive pacing rate. If thedifference between the atrial rate and the overdrive pacing rate exceedsa predetermined threshold (N_(THRESHOLD)), then the overdrive pacingrate is reset to the detected atrial rate. Otherwise, overdrive pacingcontinues at the current pacing rate.

The technique will now be described in greater detail with reference toFIG. 4. Certain steps of FIG. 4 are similar to those of FIG. 3 and,accordingly, will not be re-described in detail. Initially, at steps 300and 302, the implantable cardiac stimulation device 10 detects thecurrent intrinsic atrial rate and sets a current overdrive pacing ratebased on the detected atrial rate. Initially, the overdrive pacing rateis set to be equal to the detected atrial rate. During subsequentiterations of steps 300 and 302, the overdrive pacing rate is set to theatrial rate only if the atrial rate minus the current overdrive pacingrate exceeds N_(THRESHOLD).

At step 304, the implantable cardiac stimulation device 10 begins tocount all paced cycles (I_(RESET)) since the overdrive pacing rate wasset at step 302. At step 305, the implantable cardiac stimulation device10 paces the atria at the current overdrive pacing rate while detectingany intrinsic P-waves. At step 306, all paced cycles since pacing beganat the current rate are detected and counted (I_(CYCLES)). At step 308,the implantable cardiac stimulation device 10 also counts every group ofN consecutive paced cycles (I_(N)) wherein N is, for example, ten.Initially, the counts initiated at steps 302, 306 and 308 will be thesame. As will be seen, however, these counts may diverge from oneanother with further processing of the method steps.

At step 310, the implantable cardiac stimulation device 10 determineswhether N pacing cycles have elapsed by examining I_(N). If I_(N) equalsN, then step 312 is performed wherein the implantable cardiacstimulation device 10 determines whether at least two intrinsic P-waveshave been detected within the group of N paced cycles. If so, thecurrent overdrive pacing rate is increased at step 314 by an amount Ywherein Y is equal to, for example, 5, 10, 15, 20 or 25 ppm. Thereafter,processing returns to step 304 for additional atrial pacing at the newoverdrive pacing rate. If, at step 312, at least two intrinsic P-waveswere not detected within the group of N paced cycles, then step 316 isperformed wherein the count of N paced cycles (I_(N)) is reset such thatthe next set of N consecutive paced cycles may be counted. In thismanner, the overdrive pacing rate is increased if, and only if, at leasttwo P-waves are detected within a group of N consecutive cardiac cycles.

If, at step 310, N paced cycles have not yet elapsed (i.e., the countI_(N) is less than N), then step 318 is performed wherein theimplantable cardiac stimulation device 10 determines whether the countof paced cycles since pacing began at the current rate (I_(PACED))exceeds a rate recovery value (Z). If so, then at step 320, theoverdrive pacing rate is decreased by a pacing decrement amount W,wherein W is preset to, for example, 1, 2, 3, 4 or 5 ppm. Hence, if theoverdrive pacing rate has not been increased as a result of thedetection of at least two P-waves within a block of N cycles, then theoverdrive pacing rate is decreased to provide rate recovery.

If, at step 318, I_(CYCLES) does not exceed Z, then step 322 isperformed wherein the implantable cardiac stimulation device 10determines whether the count of paced cycles since the current rateoriginally set (I_(RESET)) in step 302 exceeds a rate calibration valueN_(RECALIBRATION). If so, then steps 300 and 302 are repeated whereinoverdrive pacing is suspended to permit detection of the intrinsicatrial rate and the overdrive pacing rate is then set based upon theintrinsic atrial rate. As noted above, within step 302, a determinationis made as to whether the difference between the intrinsic atrial rateand the overdrive pacing rate exceeds a threshold N_(THRESHOLD) and, ifnot, the overdrive pacing rate is not reset to be equal to the atrialrate. If, at step 322, I_(RESET) does not exceed N_(RECALIBRATION), thenprocessing merely returns to step 304 for additional pacing at thecurrent overdrive pacing rate.

Thus, FIG. 4 illustrates an overdrive pacing technique wherein, amongother features, (1) an overdrive pacing rate is increased only if atleast two P-waves are detected within a block of N consecutive cardiaccycles, (2) the overdrive pacing rate is decreased if the overdrivepacing rate is not increased within Z consecutive cardiac cycles, and(3) the overdrive pacing rate is periodically reset to an intrinsicatrial rate if the difference between the atrial rate and the currentoverdrive pacing rate exceeds a predetermined threshold. By increasingthe overdrive pacing rate only in response to the detection of at leasttwo P-waves within a block of N consecutive cardiac cycles, excessivelyaggressive overdrive pacing rate increases are avoided. Additionally,with appropriate selection of N, a minimum percentage of paced cyclescan be achieved on the average. For example, by setting N equal to ten,the average percentage of paced cycles will be maintained at about 90%.If more than ten percent of the cardiac cycles are intrinsic cycles,then the overdrive pacing rate is increased. Otherwise, the overdrivepacing rate is periodically decreased. Hence, an average of about 90% issustained.

With reference to FIG. 5, techniques for adaptively varying overdrivepacing characteristics are summarized. Initially, at step 400, aparticular overdrive pacing technique or algorithm is selected by theimplantable cardiac stimulation device 10. Then, at step 401,programmable values, i.e., control values, are input from a memory 402for control of the operation of the algorithm. (If the implantablecardiac stimulation device 10 is capable of performing only a singleoverdrive pacing technique, step 400 is not necessary.) Depending uponthe overdrive pacing technique, the programmable values may berepresentative of: an overdrive pacing rate, an overdrive pacing margin,a pacing cycle length, a number of pacing pulses prior to pacing cyclelength extension (Z), an amount of time prior to pacing cycle lengthextension, a number of unpaced beats prior to pacing cycle lengthextension, a rate increment magnitude (Y), a rate decrement magnitude(W), a search window duration (X), a pacemaker base rate, and a sensormodulated base rate.

At step 403, the implantable cardiac stimulation device 10 appliesoverdrive pacing pulses to the heart in accordance with the requisiteprogrammable values. While overdrive pacing is performed, theimplantable cardiac stimulation device 10 performs steps 404–412 toadjust the programmable values so as to reduce any difference between anactual degree of pacing and a target degree of pacing. Morespecifically, at step 404, the implantable cardiac stimulation device 10determines the actual degree of pacing resulting from the pacing pulses.The actual degree of pacing may be represented by a percentage of pacedbeats (determined as a function of time or as a function of cardiaccycles) or by any other appropriate factor. At step 406, a target degreeof pacing is input from a memory 408 and, at step 410, the implantablecardiac stimulation device 10 compares the actual degree of pacing withthe target degree of pacing. At step 412, the implantable cardiacstimulation device 10 adjusts the values used from memory 402 so as toreduce any difference between the actual degree of pacing and the targetdegree of pacing. The specific adjustment depends upon a particularprogrammable value being adjusted. In some cases, a value may need to beincreased so as to cause a decrease in the degree of pacing. In othercases, a value may need to be decreased so as to cause a decrease in thedegree of pacing. The direction of the adjustment and the magnitude ofthe adjustment are set so as to achieve a negative feedback loop whichconverges the actual degree of pacing to the target degree of pacing. Tothis end, routine experiments are performed to determine optimal valuesfor adjusting the various parameters to achieve the desired feedbackloop and to eliminate adjustment values, if any, which may result in apositive feedback loop causing the actual degree of pacing to deviatefrom the target degree of pacing, rather than to converge to the targetdegree of pacing. The resulting adjustment in the values may be linearor non-linear, depending upon the particular programmable values anddepending upon the amount of difference, if any, between the actualdegree of pacing and the target degree of pacing. As can be appreciated,a wide range of possible adjustments can be employed consistent with theinvention depending upon the characteristics of the overdrive pacingtechnique being implemented. In many cases, two or more programmablevalues are adjusted simultaneously. For example, both the overdrivepacing margin and the number of pacing pulses prior to a pacing cyclelength extension may be adaptively adjusted.

A first specific example of the technique of FIG. 5 will now bedescribed with reference to FIG. 6. In this specific example, theimplantable cardiac stimulation device 10 operates to maintain theoverdrive pacing rate at a rate equal to the intrinsic rate plus aprogrammable rate margin. The rate margin is adaptively varied so as tomaintain a target degree of pacing. Initially, at step 500, theimplantable cardiac stimulation device 10 inputs an initial overdrivepacing margin from a memory unit 502. The margin may be, for example,five beats per minute (bpm)—indicating that the heart is to be paced ata rate equal to the intrinsic heart rate plus five bpm. At step 504, theimplantable cardiac stimulation device 10 periodically determines theintrinsic heart rate and administers overdrive pacing pulses to theheart at a rate equal to the intrinsic rate plus the overdrive pacingmargin. For example, if the intrinsic rate is found to be 60 bpm, theimplantable cardiac stimulation device 10 overdrive paces the heart at arate of 65 ppm. If the intrinsic rate is found to increase to 80 bpm,then the overdrive pacing rate automatically increases to 85 ppm. Inthis manner, the implantable cardiac stimulation device 10 seeks tomaintain the overdrive pacing rate at a rate slightly higher than theintrinsic rate at all times.

A determination of the intrinsic rate at step 504 may be performed, forexample, by periodically deactivating overdrive pacing, therebypermitting detection of intrinsic beats from which the intrinsic heartrate is determined. In this regard, an estimate of the intrinsic heartrate may be calculated based upon the time between the detectedintrinsic beats. The greater the number of intrinsic beats that aredetected, the more precise the determination of the intrinsic heartrate.

Step 506 is periodically performed wherein the implantable cardiacstimulation device 10 counts the number of paced beats and the number ofunpaced beats until a predetermined period of time, such as 60 seconds,has elapsed. Alternatively, the implantable cardiac stimulation device10 counts the beats until a predetermined number of total beats, such as100 beats, have been counted. The implantable cardiac stimulation device10 then calculates a percentage of the number of paced beats out of atotal number of beats. In the example of FIG. 2, with nine paced beatsand one unpaced beat, the percentage of paced beats is about 90%. Atstep 508, the implantable cardiac stimulation device 10 inputs a targetdegree of pacing from a memory unit 510. The target of pacing may be,for example, 95% paced beats. At step 512, the implantable cardiacstimulation device 10 determines whether the actual percentage of pacedbeats determined at step 506 is greater than the target percentage ofpaced beats input at step 508. If so, then step 514 is performed whereinthe implantable cardiac stimulation device 10 automatically decreasesthe overdrive pacing margin by a predetermined amount, such as one ppm.If not, then step 516 is performed wherein the implantable cardiacstimulation device 10 automatically increases the overdrive pacingmargin by the predetermined amount.

Thereafter, step 504 is performed using the adjusted overdrive pacingmargin. Hence overdrive pacing may now occur at a rate of six ppm abovethe intrinsic rate or perhaps only at a rate of four ppm above theintrinsic rate. With repeated iterations of steps 504–516, the degree ofoverdrive pacing is thereby periodically, adaptively adjusted so as tomaintain the actual percentage of paced beats at an amount about equalto the target degree of pacing, e.g., at about 95%. Hence, if theinitial overdrive pacing adjustment factor was too high such thatsubstantially 100% of heart beats were paced beats, the overdrive pacingadjustment factor is decreased somewhat to permit occasional detectionof an unpaced beat. This helps ensure that the overdrive pacing rate isnot so high so as to possibly adversely affect the health of thepatient. Also, avoidance of an unnecessarily high overdrive pacing ratehelps preserve battery longevity. Moreover, in embodiments wherein theimplantable cardiac stimulation device 10 relies upon detection of anoccasional intrinsic beat so as to determine the intrinsic heart rate, areduction of the overdrive pacing rate helps ensure that intrinsic beatsare occasionally detected. On the other hand, if the actual degree ofoverdrive pacing was found to be significantly less than 95%, then theoverdrive pacing rate is increased so as to prevent too many intrinsicbeats from occurring which might trigger a tachyarrhythmia.

Although not specifically shown in FIG. 6, if, at step 512, the actualpercentage of pacing is found to be exactly equal to the target degreeof pacing, then the implantable cardiac stimulation device 10 may beconfigured to not adjust the overdrive pacing adjustment factor eitherup or down. Also, the predetermined amount by which the overdrive pacingmargin is increased may differ from that in which it is decreased. Also,the predetermined amounts may vary depending upon the current overdrivepacing rate or upon the current overdrive pacing adjustment factor. Forexample, if the overdrive pacing margin is currently set to 20 ppm, thefactor may be increased or decreased by a greater amount than if theoverdrive pacing margin was currently set to two or three ppm. Likewise,if the current overdrive pacing rate (i.e., the sum of the currentintrinsic heart rate and the current overdrive pacing margin) isparticularly high, then the predetermined amounts may also be relativelyhigh. Also, note that the overdrive pacing margin may, at times, benegative. As can be appreciated, a wide range of alternatives may beprovided consistent with the principles of the invention.

Another specific example of the technique of FIG. 5 will now bedescribed with reference to FIG. 7. In this specific example, theimplantable cardiac stimulation device 10 performs a dynamic atrialoverdrive technique wherein detection of a single P-wave triggers animmediate, significant increase in the overdrive pacing rate andwherein, after an increase, the pacing cycle length is periodicallyextended to gradually reduce the overdrive pacing rate. Morespecifically, the dynamic atrial overdrive technique operates asfollows. The implantable cardiac stimulation device 10 monitors theatria of the heart and detects P-waves and, in response to detection ofa single P-wave, increases the overdrive pacing rate by a programmableincrement value which depends upon whether the current overdrive baserate is within: 1) a lower rate overdrive (LRO) regime of between 25 and59 ppm, 2) a middle rate overdrive regime (MRO) of between 60 and 149ppm, or 3) an upper rate overdrive (URO) of between 150 and 185 ppm.

Within the LRO regime, the implantable cardiac stimulation device 10increases the overdrive pacing rate with each sensed P-wave by an LROincrement programmable value, e.g., 5, 10, 15, 20 and 25 ppm. Within theURO regime, the implantable cardiac stimulation device 10 increases theoverdrive pacing rate with each sensed P-wave by a URO incrementprogrammable value, e.g., 5 or 10 ppm. (Typically, the LRO incrementvalue is programmed to a high value, such as 25 ppm, whereas the UROincrement is programmed to a low value such as 5 ppm.) Within the MROregime, the implantable cardiac stimulation device 10 increases theoverdrive pacing rate with each sensed P-wave by an MRO increment whichis a blended value between the LRO increment and the URO increment. TheMRO increment is equal to the LRO increment when the base rate is 60ppm. The MRO increment varies gradually when the base rate is in therange of 60 ppm to 150 ppm until the increment is equal to the UROincrement when the base rate is equal to 150 ppm.

The implantable cardiac stimulation device 10 also exploits a dynamicrate recovery technique wherein the overdrive base rate is decreased ifa predetermined number of pacing cycles occur without any detectedP-waves. The predetermined number of cycles and the amount of thedecrease are both programmable. The amount of the decrease variesdepending upon whether the base overdrive pacing rate is within one oftwo regimes.

The specific operation of the implantable cardiac stimulation device 10within the various regimes is described with reference to the followingexamples.

As an example of operation within the LRO regime, if the currentoverdrive pacing rate is 45 ppm, the LRO increment value is 5 ppm, and aP-wave is sensed, the current overdrive pacing rate is immediatelyincreased to 50 ppm. If the P-wave arises from intrinsic atrial activityoccurring at a rate of 53 bpm, then a second P-wave will be detectedbefore a paced beat can be generated (because the overdrive base rate isstill below the intrinsic rate). Hence, another P-wave is detected andthe overdrive pacing rate increases to 55 ppm.

As another example of operation within the LRO regime, if the currentoverdrive pacing rate is 55 ppm, the LRO increment is 25 ppm, the UROincrement is 5 ppm, and the patient experiences an SVT at 160 bpm, thenthe overdrive pacing rate increases by 25 ppm with each sensed atrialbeat until the overdrive pacing rate exceeds 60 ppm. Then, any furtherincrements begin at slightly less than the LRO increment of 25 ppm andare gradually reduced to the URO increment of 5 ppm when the overdrivepacing rate exceeds 150 ppm.

To provide rate recovery, the implantable cardiac stimulation device 10counts the number of pacing pulses delivered at a current overdrivepacing rate and, if the number of cycles exceeds a threshold valueN_(MAX), the implantable cardiac stimulation device 10 decreases theoverdrive pacing rate by increasing a pacing cycle length (CL) equal tothe amount of time between individual pacing pulses. N_(MAX) ispreferably programmable within a range of 1 to 32 cycles. Thus, ifN_(MAX) is programmed to 10 cycles and the overdrive pacing rate hasremained constant for 10 cycles, then the CL is increased by aprogrammable rate recovery value. In this manner, so long as nointrinsic activity is detected, the overdrive pacing rate graduallydecreases. Whenever intrinsic atrial activity is sensed, the counterassociated with N_(MAX) is reset and, in accordance with the techniquesalready described, the overdrive pacing rate is incremented. Exemplaryprogrammable CL increment values are:

Milliseconds/Cycle  6; 13  6; 19 13; 19 19; 25

As noted, an increase in the pacing cycle length causes a correspondingdecrease in the overdrive pacing rate. In the foregoing, the first valuerepresents the increase in CL in milliseconds per cycle to be used ifthe current base rate is over 100 ppm. The second value represents theincrease in CL in milliseconds per cycle to be used if the current baserate is 100 ppm or less. Thus, two base CL increment regimes are used.

In a specific rate recovery example, if the current overdrive pacingrate is 102 ppm, the intrinsic atrial rate is 90 ppm, and the dynamicrate recovery values are programmed to 6; 19 milliseconds/cycle, thenthe pacing cycle length decreases after every N_(MAX) as follows:

-   -   (1) 595 milliseconds (101 ppm)    -   (2) 601 milliseconds (100 ppm)    -   (3) 620 milliseconds (97 ppm)    -   (4) 639 milliseconds (94 ppm)    -   (5) 658 milliseconds (91 ppm)

Thus, the implantable cardiac stimulation device 10 employs a dynamicatrial overdrive technique which increases an overdrive base rate verypromptly in response to detection of intrinsic atrial activity (i.e.,P-waves) and provides a rate recovery technique for reducing theoverdrive pacing rate when overdrive pacing is no longer needed. Thedegree of increment or decrement to the overdrive pacing base ratedepends, as described above, upon the current base rate regime.Additional variations to the overdrive pacing rate may be based upondetection of premature atrial contractions (PACs) or other intrinsicevents.

In the technique of FIG. 7, N_(MAX) is adaptively varied to maintain atarget degree of pacing. Initially, at step 600, the implantable cardiacstimulation device 10 inputs both an initial pacing cycle length (CL)and N_(MAX) from a memory unit 602. CL may be, for example, one second(corresponding to an overdrive pacing rate of 60 ppm) and N_(MAX) maybe, for example, initially set to ten.

At step 604, the implantable cardiac stimulation device 10 repeatedlyadministers overdrive pacing pulses to the heart in accordance with thedynamic atrial overdrive algorithm described above wherein the CL isautomatically extended every N_(MAX) pulses to allow occasionaldetection of intrinsic heart beats or other intrinsic activity. Whilestep 604 is performed, the implantable cardiac stimulation device 10additionally performs steps 606–616 as follows. The implantable cardiacstimulation device 10 counts the number of paced beats and unpaced beatsfor either a predetermined period of time or a predetermined number ofpulses and then calculates the percentage of paced beats at step 606. Atarget degree of pacing is input at step 608 from a memory 610 and, atstep 612, the implantable cardiac stimulation device 10 determineswhether the actual percentage of pacing is greater than the targetpercentage of pacing. If so, then N_(MAX) is decreased by apredetermined amount, such as one cycle, at step 614. If not, thenN_(MAX) is increased by a predetermined amount, such as one cycle, atstep 616. Thereafter, the overdrive pacing performed by the implantablecardiac stimulation device 10 during step 604 is performed using theadjusted value for N_(MAX). With repeated iterations of steps 604–616,the actual degree of pacing is maintained substantially at or near thetarget degree of pacing so as to prevent excessive overdrive pacingwhile still minimizing the number of non-paced beats. In this regard,N_(MAX) is decreased when the actual percentage of pacing is greaterthan the target percentage of pacing so as to permit a more promptdetection of an intrinsic pulse from which a new intrinsic heart rate isdetermined. By permitting a more prompt detection of a next intrinsicbeat, the overdrive pacing rate can thereby be adjusted in accordancewith the dynamic atrial overdrive algorithm to more closely match theintrinsic rate. In contrast, by increasing N_(MAX) if the actualpercentage of pacing is found to be less than 95%, a greater amount oftime elapses prior to detection of a next intrinsic pulse, therebydelaying readjustment of the overdrive pacing rate. This may result in agenerally higher overdrive pacing rate. In any case, regardless ofwhether the adjustments to N_(MAX) result in an increase or decrease inthe overall average overdrive pacing rate, the adjustments to N_(MAX)will typically operate to maintain the percentage of paced beats atabout the target percentage and the advantages set forth above areachieved.

In another specific example of the technique of FIG. 5, the implantablecardiac stimulation device 10 employs a technique for modulating thebase rate of the implantable cardiac stimulation device 10 based uponcircadian rhythms of the patient. The technique for modulating the baserate is described in U.S. Pat. No. 5,476,483 to Bornzin et al. which isincorporated by reference herein. Briefly, in accordance with thetechnique of the Bornzin et al. patent, the base rate associated with atransfer function of a rate-responsive cardiac pacemaker is modulated.Activity sensor measurements are used to derive activity variancemeasurements, which in turn are used to modulate the base pacing rate.In one embodiment, a histogram is used to store activity variancemeasurements collected over a period of about one week. A histogram isused to derive an activity variance threshold, which is compared tocurrent activity variance measurements to determine if the patient isasleep. If the patient is deemed to be asleep, the pacing rate is set toa rate that comfortably meets the low metabolic demands of the patientduring sleep. In alternative embodiments, the activity variancemeasurements are applied to a base rate slope to modulate the basepacing rate.

Thus, the base rate can be modulated and as such, the percentage of timethat underlying, intrinsic P-waves are detectable during a search periodcan be adjusted by adaptively adjusting the base rate in accordance withthe adaptive techniques described above. With proper selection ofappropriate adaptive adjustment values (when the implantable cardiacstimulation device extends an atrial escape interval to permit detectionof an underlying, intrinsic P-wave), the adjusted base rate will limitthe extension of the escape interval so that the base rate is in facthigher than the underlying atrial rate. Accordingly, there will be few,if any, emerging P-waves. More specifically, parameters sleep_rate andBPR_slope within Equation 9 of the Bornzin et al. patent are adaptivelyadjusted so as to achieve a target degree of pacing. Increasingsleep_rate and BPR_slope has the effect of increasing the base rate andthus increasing the percentage of atrial pacing.

What has been described are various techniques for pacing multiple sitesin a patient's heart using overdrive pacing including techniques foradaptively adjusting overdrive pacing programmable values so as tomaintain a desired or target degree of overdrive pacing, or to maintainany other desirable characteristic of the overdrive pacing. Theoverdrive pacing techniques have been described primarily with referenceto flow charts illustrating steps performed by components of animplantable cardiac stimulating device. Each method step of the flowcharts additionally represents a functional component for performing themethod step. The functional component may comprise individual hardwaredevices or may comprise software components. In some cases a singlefunctional component will perform two or more method steps. In othercases two or more functional components in combination will perform asingle method step. In general, the techniques described herein may beimplemented using any appropriate technology such as hardware, software,firmware, or the like.

The examples described herein are merely illustrative of the inventionand should not be construed as limiting the scope of the invention.Accordingly, the scope of the present invention is defined by thefollowing claims.

1. A method for overdrive pacing a patient's heart using an implantablecardiac stimulation device connected to a plurality of electrodesimplanted in a patient's heart, the method comprising: sensing intrinsicatrial depolarizations of the patient's heart; determining an overdrivepacing rate based on the intrinsic atrial depolarizations; and overdrivepacing the heart via the plurality of electrodes at the overdrive pacingrate, wherein the plurality of electrodes define at least twoindependent electrode configurations, and wherein each of the electrodeconfigurations independently stimulates the heart; wherein sensingcomprises individually sensing depolarizations from the plurality ofelectrodes and determining comprises determining the overdrive pacingrate according to the highest sensed depolarization rate.
 2. The methodof claim 1 wherein the plurality of electrodes are implanted in theatria of the patient's heart.
 3. The method of claim 1 wherein theplurality of electrodes are implanted in the right atrium of thepatient's heart.
 4. The method of claim 1 wherein the plurality ofelectrodes are implanted proximate to the left atrium of the patient'sheart.
 5. The method of claim 1 wherein a first electrode is implantedin the right atrium of the patient's heart and a second electrode isimplanted in the coronary sinus of the patient's heart.
 6. The method ofclaim 1 wherein a first electrode is implanted in the right atrium ofthe patient's heart and a second electrode is implanted in the leftatrium of the patient's heart.
 7. The method of claim 1 whereinoverdrive pacing comprises sequentially supplying stimulation pulses tothe electrodes implanted in the patient's heart.
 8. An implantablecardiac stimulation device configured for overdrive pacing a patient'sheart via a plurality of electrodes that are adapted to be implanted ina patient's heart, the device comprising: at least one sensing circuitthat is operative to sense intrinsic atrial depolarizations of thePatient's heart via one or more of the plurality of electrodes; acontroller that is operative to determine an overdrive pacing rate basedon the sensed atrial depolarizations; and at least one pulse generatorthat is controlled by the controller to generate pacing pulses at theoverdrive pacing rate for delivery to the heart via the plurality ofelectrodes; wherein the plurality of electrodes define at least twoindependent electrode sets, and wherein each of the electrode setscomprises at least one electrode and wherein each of the electrode setsreceives the pacing pulses from the pulse generator and independentlystimulates the heart; wherein the at least one sensing circuit comprisesa first and second sensing circuit for individually sensingdepolarizations from the plurality of electrodes; and wherein thecontroller determines the overdrive pacing rate according to the highestsensed depolarization rate.
 9. The device of claim 8 wherein theplurality of electrodes are adapted to be implanted in the atria of thepatient's heart.
 10. The device of claim 8 wherein the plurality ofelectrodes are adapted to be implanted in the right atrium of thepatient's heart.
 11. The device of claim 8 wherein the plurality ofelectrodes are adapted to be implanted proximate to the left atrium ofthe patient's heart.
 12. The device of claim 8 wherein a first electrodeis adapted to be implanted in the right atrium of the patient's heartand a second electrode is adapted to be implanted in the coronary sinusof the patient's heart.
 13. The device of claim 8 wherein a firstelectrode is adapted to be implanted in the right atrium of thepatient's heart and a second electrode is adapted to be implanted in theleft atrium of the patient's heart.
 14. The device of claim 8 whereinthe at least one pulse generator comprises a first and a second pulsegenerator for sequentially supplying stimulation pulses to theelectrodes.
 15. The device of claim 8 wherein the at least one pulsegenerator is operative to simultaneously supply stimulation pulses tothe electrodes.
 16. The device of claim 8 wherein the controllercomprises: means for detecting intrinsic heart beats via the pluralityof electrodes arising during the overdrive pacing; means for increasingthe overdrive pacing rate by a determined rate increment if at least twointrinsic heart beats are detected within a first determined searchperiod; and means for decreasing the overdrive pacing rate by adetermined rate decrement if at least two intrinsic heart beats are notdetected within a second determined search period.
 17. An implantablecardiac stimulation device configured for overdrive pacing a patient'sheart via a plurality of electrodes that are adapted to be implanted ina patient's heart, the device comprising: at least one sensing circuitthat is operative to sense intrinsic atrial depolarizations of thepatient's heart via one or more of the plurality of electrodes; acontroller that is operative to determine an overdrive pacing rate basedon the sensed atrial depolarizations; and at least one pulse generatorthat is controlled by the controller to generate pacing pulses at theoverdrive pacing rate for delivery to the heart via the plurality ofelectrodes; and wherein the plurality of electrodes define at least twoindependent electrode sets, and wherein each of the electrode setscomprises at least one electrode and wherein each of the electrode setsreceives the pacing pulses from the pulse generator and independentlystimulates the heart; and wherein the controller further comprises meansfor determining a sinus rate and means for setting the overdrive pacingrate to be equal to the sinus rate.
 18. A system for controllingoverdrive pacing of a patient's heart, the system comprising: means forsensing intrinsic atrial depolarizations; means for determining anoverdrive pacing rate based on the intrinsic atrial depolarizations;means for overdrive pacing the heart at the overdrive pacing rate and ata plurality of independent stimulation sites; means for determining asinus rate; and means for setting the overdrive pacing rate to be equalto the sinus rate.